Flue gas desulfurization of combustion gas streams with an injected dry sorbent for sulfur oxide removal (sulfur oxides collectively being referred to as SOX, which comprises SO2 and SO3) provides an increasingly favorable alternative to the traditional wet scrubbing techniques, using lime or limestone as the SO2-reactive agent, or spray drying systems, using lime as the preferred SO2-reactive agent.
Dry sorbent flue gas desulfurization offers advantages of operational simplicity and reliability, competitive reagent costs, high SOX-removal efficiencies, attractive equipment and labor costs, and lower water consumption over the widely used wet scrubbing desulfurization procedures currently in use. Dry sorbent flue gas desulfurization is particularly well-suited for retrofitting existing combustion flue gas facilities such as coal-fired electric power utility plants in need of upgraded SOX pollution control.
In dry sorbent flue gas desulfurization, a suitable sorbent such as trona or nahcolite is injected as a dry powder into a SOX-containing combustion flue gas stream where it becomes fluidized and entrained in the flowing hot gas stream and is then collected downstream, usually along with entrained fly ash in the hot flue gas stream, via particulate solids control equipment such as electrostatic precipitators or fabric filtration (baghouse filters) collection devices. The SOX removal occurs while the particulate sorbent is carried in the hot flue gas stream and also while the collected sorbent is still in contact with the flowing gas stream. The collected solids mixture of sorbent-SOX reaction product and fly ash is periodically removed for disposal.
Trona, a naturally-occurring mineral form of sodium sesquicarbonate, is especially favored as a dry injection sorbent for SOX control. Commercial acceptance of the use of trona as a dry injection flue gas desulfurization agent is not only dependent on SOX-removal reaction efficiencies of trona but also on the overall cost of trona per unit SOX removed.
Many prior art patents disclose techniques for treating trona ore, to remove impurities like insoluble oil shale from the naturally-occurring trona ore, but the teachings of these patents are typically directed to the purification and calcination of trona ore used to make soda ash (Na2CO3).
U.S. Pat. No. 3,244,476 of Smith (Intermountain Research & Development Corp.) describes a process for separation of the soluble fraction (sodium carbonate) from the insoluble fraction (shale, silica, etc.), in which crude trona is first calcined, then crushed and the smaller-sized calcined trona fraction is preferentially separated from the larger-sized sintered shale fraction, to recover a soda ash with low insolubles content.
U.S. Pat. No. 3,819,805 of Graves et al. (Allied Chemical Corp.) describes a process in which mined and crushed trona is separated from the insoluble shale impurities via a color separation technique. The trona, which is primarily white, tan and light brown in color, is separated from the dark brown, green gray and black pieces, by an optical sorting procedure on the crushed, wetted ore pieces that are preferably 0.5 in. to 6 in. in diameter. The optical sorting may be based on reflectivity, transmission or absorption of electromagnetic radiation of selected wavelength(s) or on a variable standard of fluorescence. The patent also notes that crushing trona ore tends to concentrate the insolubles in the finer-sized fraction (col. 3, lines 1-7). The trona fraction of white and light brown pieces is then subjected to calcination to recover a sodium carbonate or soda ash product.
U.S. Pat. No. 4,341,744 of Brison et al. (Stauffer Chemical Co.) describes a process for producing soda ash (sodium carbonate) by crushing the trona ore to a maximum particle size of 4 mm and removing fines, separating the particles into at least two fractions by electrostatic separation to obtain a beneficiated trona fraction, and calcining the beneficiated fraction to convert the trona to soda ash.
U.S. Pat. No. 4,375,454 of Imperato et al. (Intermountain Research & Development Corp.) describes a process for beneficiating trona or nahcolite ore by crushing the trona ore to a particle size less than about 6.7 mm, calcining the crushed ore to give obtain at least a surface coating of sodium carbonate, separating off the fines and then subjecting sized particles greater than 0.212 mm to a high voltage gradient separation to recover an electrostatically-enriched soda ash fraction. The beneficiated soda ash may be sold as is or used as feed to a monohydrate soda ash process.
Several patents assigned to Environmental Projects, Inc. describe multistep beneficiation processes applicable to the treatment of trona ore to remove impurities.
U.S. Pat. No. 5,470,554 of Schmidt et al. (Environmental Projects) describes a multistep process for the beneficiation of saline minerals, e.g., trona ore, by crushing and sizing the ore via separate steps of density separation of a first ore portion, electrostatic separation of a second portion and magnetic separation of a third portion. In the case of trona, the Schmidt et al. '554 patent teaches that the ore impurity shortite may be separated from trona via density separation and that shale, mudstone and pyrite impurities may be separated via electrostatic separation. The patent also teaches that the trona ore may be dried prior to the various separation steps, to effect better separation of the impurities. The purified trona is said to be particularly suited for use in manufacture of caustic soda for alumina production and in the glass industry.
U.S. Pat. No. 6,092,665 of Schmidt et al. (Environmental Projects) is a continuation-in-part of Schmidt et al. '554, summarized above. The Schmidt et al. '665 patent differs from Schmidt et al. '554 in that the '665 patent includes an additional step of calcining the trona ore and subsequently separating a first portion of impurities by density separation.
U.S. Pat. No. 5,736,113 of Hazen et al. (Environmental Projects) describes a process for removing impurities from trona via an electrostatic separation method carried out at a temperature between about 25° C. and about 45° C. The trona beneficiation may also include other types of separation operations, such as magnetic separation, density separation, and size separation. The patent also teaches that the trona ore may be dried prior to the various separation steps, to effect better separation of the impurities. The Hazen et al. '113 patent states that the beneficiated trona may be calcined to produce sodium carbonate.
U.S. Pat. No. 5,911,959 of Wold et al. (Environmental Projects) describes multistep processes for the treatment of trona ore having insoluble impurities, by first calcining the crushed ore and then subjecting the calcined trona to dry or wet separation steps or both. The dry separation processes may include density separation, magnetic separation, or electrostatic separation steps, and the wet separation processes may include introduction of the calcined trona to a saturated sodium carbonate brine solution to effect crystallization of sodium carbonate monohydrate. Sodium carbonate is recovered as the product in these multistep processes.
U.S. Pat. No. 6,173,840 of Pruszko et al. (Environmental Projects) discloses a beneficiation process applicable to purification of saline minerals, trona being preferred, and the process utilizes a magnetic separation technique. Other separation techniques may be employed in conjunction with the magnetic separation, including colorimetric separation, density separation and size separation. The trona is preferably calcined prior to carrying out the magnetic separation and density separation. Apart from the optional calcination, the patent also teaches that the trona ore may be dried prior to the various separation steps, to effect better separation of the impurities. The Pruszko et al. '840 patent mentions that the shortite separated and recovered as an impurity from trona may be used in the treatment of flue gases from removal of sulfur or acidic gases from the flue gas (sentence bridging cols. 8-9.)
U.S. Pat. No. 7,473,407 and U.S. Pat. No. 7,770,735 of Phillip et al. (Solvay Chemicals) describe beneficiation processes for trona ore in which crushed trona ore is first crushed and dried. The dried trona ore is then separated into a first (larger) size fraction and second (smaller) size fraction, the latter having reduced impurity levels. The first fraction is subjected to magnetic separation to remove impurities from this first ore fraction. The two beneficiated trona ore fractions may then be combined.
The trona patents summarized above do not mention use of trona for dry injection pollution control applications such as flue gas desulfurization. However, as mentioned above, an important and growing market for trona ore is in flue gas desulfurization, via injection as a particulate solid into combustion gas streams, for removal of SOX before the gas stream is released into the atmosphere. The use of trona in dry injection flue gas desulfurization is described in the prior art, in both older and more recent patents.
U.S. Pat. No. 4,555,391 of Cyran et al. (Intermountain Research & Development Corp.) describes a dry injection desulfurization process that uses a dry soda-type compound such as trona or nahcolite as the sorbent to desulfurize a SO2-containing flue gas stream and recycles a portion of the spent sorbent to increase sorbent utilization efficiency. U.S. Pat. No. 4,588,569 of Cyran et al. (Intermountain Research & Development Corp.) describes a dry injection desulfurization process that uses a calcined soda ash sorbent, obtained from calcination of a NaHCO3-containing compound like sodium bicarbonate or sodium sesquicarbonate (trona), to desulfurize a SO2-containing flue gas stream at a temperature of 100° C. to about 175° C.
U.S. Patent Publication No. 2005-0201914 of Ritzenthaler (American Electric Power Co.) describes a dry injection process for removing acid gases, including sulfur trioxide and sulfuric acid, from flue gas streams using a dry sodium sorbent. The sodium sorbent may be sodium sesquicarbonate, sodium carbonate-bicarbonate, trona or mechanically refined ore, which is injected into the flue gas stream and calcined to soda ash which reacts with the strong acid components in the flue gas.
U.S. Pat. No. 7,481,987 of Maziuk, Jr. (Solvay Chemicals) describes a dry injection desulfurization process in which sulfur trioxide (SO3) is removed from a flue gas stream, using a sorbent that is sodium sesquicarbonate (preferably trona) or sodium bicarbonate or soda ash. U.S. Pat. No. 7,531,154 of Maziuk, Jr. (Solvay Chemicals) describes a dry injection desulfurization process in which sulfur dioxide (SO2) is removed from a flue gas stream at a temperature of about 600° F. to about 900° F., using trona as the sorbent. Both patents disclose that Solvay's T-200® trona product is a suitable trona source and that T-200® trona is a mechanically refined trona ore of about 97.5% sodium sesquicarbonate having a mean particle size of about 24-28 microns.
The present invention provides a method for producing high purity dry trona product from impure trona ore deposits, to provide an uncalcined particulate trona product that is especially well suited for removing pollutants from gas streams via dry injection flue gas desulfurization.